Infrared (IR) spectroscopy typically involves the transmission of light of a range of infrared frequencies through a sample, resulting in the detection of the absorption of certain frequency components by certain materials thereby indicating the presence of those components in the sample. It has been found that a "mid-IR" range of frequencies, corresponding to 2.5-25 microns in wavelength or 4000-400 in wavenumbers, is particularly useful for analyses of oils, lubricants, and beverages, particularly dairy products.
Remote infrared spectroscopic monitoring using optical fibers is useful in spectroscopy, as discussed, for example, in U.S. Pat. No. Re. 33,789 to Stevenson, U.S. Pat. No. 5,076,243 to Bornstein, U.S. Pat. No. 5,239,176 to Stevenson, U.S. Pat. No. 4,852,967 to Cook, and U.S. Patent No. 5,585,634 to Stevenson et al. The material being analyzed or monitored may be gaseous, liquid, or solid, and sampling may be readily performed outside the sample compartment of a conventional spectrometer thus permitting in situ, real-time spectroscopic measurements and eliminating the need to transport a sample to the spectrometer.
The components for a transmitting light in the mid-IR through a traditional transmission cell containing a sample are expensive, including expensive antireflection-coated germanium/zinc selenide lenses to collimate the light directed to the sample and the light transmitted from the sample. Moreover, such components are difficult to handle, requiring relatively precise alignment, but being prone to scratching and breaking.
Other solutions such as the use of parabolic mirrors also required relatively precise alignment, which presented problems in an industrial process environment where mid-IR spectroscopy can be put to good use.
Non-imaging concentrators, comprising highly reflective internal conical or compound parabolic surfaces have been used to collimate IR light in spectroscopy, for example, the "System Having Non-Imaging Concentrators for Performing IR Transmission Spectroscopy," disclosed in U.S. Pat. No. 5,254,858, issued Oct. 19, 1993, to Wolfman et al. That system, however, calls for the use of two concentrators tapered towards each other to collimate a light source and does not appear adapted to an in-process configuration.